Magneto inductive flow measuring devices utilize the principle of electrodynamic induction for volumetric flow measurement and are disclosed in a large number of publications. Charge carriers of the medium moved perpendicularly to a magnetic field induce a measurement voltage in measuring electrodes arranged essentially perpendicularly to the flow direction of the medium and perpendicularly to the direction of the magnetic field. The measurement voltage induced in the measuring electrodes is proportional to the flow velocity of the medium averaged over the cross section of the measuring tube, thus proportional to the volume flow rate. If the density of the medium is known, the mass flow in the pipeline, respectively in the measuring tube, can be determined. The measurement voltage is usually tapped via a measuring electrode pair, which is arranged in the region of maximum magnetic field strength relative to the coordinate along the measuring tube axis and where, thus, the maximum measurement voltage is to be expected. The electrodes are usually galvanically coupled with the medium; known, however, also, are magneto inductive flow measuring devices with contactless, capacitively coupling electrodes.
The measuring tube can be manufactured, in such case, either of an electrically conductive, non-magnetic material, e.g. stainless steel, or of an electrically insulating material. If the measuring tube is manufactured of an electrically conductive material, then it must be lined in the region coming in contact with the medium with a liner of an electrically insulating material. The liner is composed, depending on temperature and medium, for example, of a thermoplastic, thermosetting or elastomeric, synthetic material. Known, however, are also magneto inductive flow measuring devices equipped with a ceramic lining.
An electrode can be subdivided essentially into an electrode head, which comes at least partially in contact with a medium, which flows through the measuring tube, and an electrode shaft, which is encapsulated almost completely in the wall of the measuring tube.
Besides the magnet system, the electrodes are the central components of a magneto inductive, flow measuring device. In the embodiment and arrangement of the electrodes, it is to be heeded that they can be mounted as simply as possible into the measuring tube and that subsequently in measurement operation no sealing problems occur; moreover, the electrodes should provide a sensitive and simultaneously low-disturbance registering of the measurement signal.
Besides the measuring electrodes serving for tapping a measurement signal, often additional electrodes in the form of reference, or grounding, electrodes are installed in the measuring tube, which serve to measure an electrical, reference potential or to detect partially filled measuring tubes or to register the temperature of the medium by means of an installed temperature sensor.